Traction motors for prior art electrically driven vehicles have usually been direct current series motors which have an approximate inverse relation between torque and speed, the motors therefore providing the high torque whenever it is required at start but high speed and low torque under running conditions. However the power input is the product of current and voltage while power output is measured in foot pounds per minute. Consequently on start and at low speed series motors are particularly inefficient, they draw extremely high currents and if the power source is a battery bank, they can and often do damage the batteries. High motor efficiency at all speeds is a most desirable feature for battery driven vehicles if a reasonable range of travel is to be achieved.
A printed circuit motor is a highly efficient motor of relatively low weight, and has a low inductance. The concept of the series motor for traction purposes is completely unsuited for use with a printed circuit motor. The torque of a printed circuit motor is a function of the current (being approximately proportional), and one of the objects of this invention is to provide a system which will match available voltage to the motor voltage requirement, (although the invention is not necessarily limited to use with a printed circuit motor).
If for example a motor torque is maintained constant and speed increases, the horsepower output increases, and thus there is a need to provide voltage control means which will increase the voltage. However, in a vehicle it is required to vary torque (current) as well as voltage, and desirable to avoid "jumps" as further batteries are switched in, and a further object is to provide means for achieving this smooth and efficient control of motor torque.